Ramban: He added that, a. due to what Avraham just did, He swore by His Name, and b. that his children would capture their enemies' territory. In other words, no sin would ever cause his children to be totally wiped out, and that nothing would ever cause them to fall into the hands of their enemies, never to arise again.
Seforno: Hashem swore by His Name that He would increase his children. 1
Presumably, like the Ramban, he explains that the promise to increase Avraham's children Hashem now made by His Name, which rendered it unconditional, in which case it was permanent and not subject to change.
What did Hashem mean when He said "Bi Nishba'ayi"?
Rashi: Refer to 32:13:2:1.